Specialised downhole pumps are used in the hydrocarbon exploration and production industry in various applications, and in particular for the production of hydrocarbons to surface from significant wellbore depths. There are several types of downhole pump in use, including Electrical Submersible Pumps (ESPs) and Progressive Cavity Pumps (PCPs). An ESP is typically located at the bottom of the production tubing, and comprises a downhole electric motor powered and controlled from surface by a power cable which connects to the wellhead. ESPs are highly efficient pumps capable of high production rates, and are particularly well-suited to the production of lighter crude oils, and are less capable with heavy crudes.
A PCP, like an ESP, is typically attached to the bottom end of a production tubing. A PCP comprises a rubber stator having a helical internal profile which mates with a rotor having an external screw profile. The rotor is connected to a rotating shaft, which extends through the production tubing and is driven by a surface motor. PCPs are normally specified for their ability to produce heavy crudes.
Downhole pumps are sensitive to sands and other abrasive solids being present in the production fluid. The amount of sand which is produced from a well depends on characteristics of the formation, and various methods are used to control sand production. However, it is common for some amount of sand or abrasive solids to be present in the production fluid. ESPs are particularly sensitive to sand presence due to the nature of their internal components.
With many production systems which use a downhole pump, problems can arise when the pump is shut down after a period of pumping fluid up the production tubing to surface. On pump shutdown, flow ceases very quickly as the fluid levels in the production bore and the annulus equalise. Gravity acting on the sand particles present in the column of fluid above the pump (which could be several thousand metres) causes the sand and any other solids to fall back towards the pump. Due to the complex configuration of the interior features of the pump, there is no direct path for the sand to pass through the pump, and therefore it tends to settle on top of the pump. This can cause the pump to become plugged. When production operations are resumed, a higher load is required to start the pump and push the plug of sand up from the pump. In some cases this can cause motor burnout in an ESP or breaking of the rotor shaft of PCP. Such failure of the downhole pump requires work-over involving pull-out and reinstallation of the completion. This is an expensive and time-consuming operation.
It is amongst the aims and objects of the invention to provide a downhole apparatus and method which addresses the above-described deficiencies of downhole pump systems.
Further aims and objects will become apparent from reading the following description.